Prior art steam generating devices comprise a steam vessel receiving water, provided with an electric heating device for heating the water. When current is supplied to the heating device, it generates heat which is transferred to the water, thus creating steam.
The current in the heating device is controlled by a controller for keeping the temperature of the steam at a desired level. To this end, prior art steam generating devices comprise a temperature sensor associated with the heating device or in contact with the steam; if the sensor indicates a temperature above a predetermined level, the heating current is switched off, if the sensor indicates a temperature below a predetermined level, the heating current is switched on.
Some prior art devices also comprise a pressure sensor. EP-0.595.292 discloses a steam generating device provided with a pressure sensor as a safety device: if the pressure becomes too high, the heating current is switched off.
EP-0.843.039 discloses a steam generating device provided with a pump for adding water to the steam vessel. The water is added on the basis of a signal from a pressure sensor.
US-2004/0.040.185-A1 discloses a steam generating device where control is aiming to maintain the steam pressure at a constant level. Instead of using a pressure sensor, as earlier art had done, it proposes to use a temperature sensor and perform control on the basis of the temperature value as detected by the temperature sensor, based on recognition of the existence of a one-to-one relationship between temperature and pressure.
The present invention relates specifically to a steam generating device capable of providing steam at a variable output rate (expressed, for instance, in gram/min). This issue is not addressed by the above-mentioned publications.
EP-0.390.264-B1 discloses a steam generating device with variable steam output rate, wherein the steam output rate is controlled by setting the duty cycle of the heating current, based on the assumption that generating a certain amount of steam requires a certain amount of energy.